Method for producing superabsorbents

Abstract

The present invention relates to a process for the continuous production of superabsorbents, which comprises coating superabsorbent particles by spray application of a surface postcrosslinker solution, thermally surface postcrosslinking the coated superabsorbent particles in contact dryer 1, cooling the thermally surface postcrosslinked superabsorbent particles in contact dryer 2, and metering particulate solids into the product stream between contact dryer 1 and contact dryer 2.

Description

[Technical Field]

[0001] The present invention relates to a process for the continuous production of superabsorbents, which comprises coating superabsorbent particles by spray application of a surface postcrosslinker solution, thermally surface postcrosslinking the coated superabsorbent particles in contact dryer 1, cooling the thermally surface postcrosslinked superabsorbent particles in contact dryer 2, and metering particulate solids into the product stream between contact dryer 1 and contact dryer 2. [Background technology]

[0002] Superabsorbents are used to make diapers, tampons, sanitary napkins and other hygiene articles, but are also used as moisture-retaining agents in commercial horticulture. Superabsorbents are also called water-absorbing polymers.

[0003] The production of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pp. 71-103.

[0004] Performance characteristics, such as gel bed permeability (GBP) and 2 It is common to surface postcrosslink superabsorbent particles to improve their absorbency under a pressure of 49.2 g / cm (AUL 0.7 psi). This increases the level of crosslinking on the particle surface, resulting in a 2 This allows the absorbency at a pressure of 0.7 psi (AUL) and the centrifuge retention capacity (CRC) to be at least partially separated. This surface postcrosslinking can be carried out in an aqueous gel phase. However, it is preferred to surface-coat dried, crushed, and sieved superabsorbent particles (base polymer) with a surface postcrosslinker and then thermally surface-postcrosslink them. Crosslinkers suitable for this purpose are compounds capable of forming covalent bonds with at least two carboxylate groups of the superabsorbent particles. Summary of the Invention [Means for solving the problem]

[0005] It was an object of the present invention to provide an improved process for coating surface postcrosslinked superabsorbent particles with a particulate solid.

[0006] This object was achieved by a process for continuously producing superabsorbents by coating superabsorbent particles by spray application of a surface postcrosslinker solution, thermally surface postcrosslinking the coated superabsorbent particles in contact dryer 1, and cooling the thermally surface postcrosslinked superabsorbent particles in contact dryer 2, wherein the surface postcrosslinked superabsorbent particles are further coated with a particulate solid, which is metered into the product stream between contact dryer 1 and contact dryer 2, contact dryer 2 having two horizontal shafts with mixing devices, the speed of the mixing devices corresponding to a Froude number of 0.005 to 0.25.

[0007] Examples of contact dryers suitable for the continuous process of the present invention include paddle dryers and disk dryers. In contact dryers, the material to be dried is moved and re-laid by a dynamic device along a heated surface. Contact dryers can also be used for cooling.

[0008] The speed of the mixing tool preferably corresponds to a Froude number of 0.01 to 0.21, more preferably 0.02 to 0.18, most preferably 0.04 to 0.15.

[0009] For a mixer with horizontally mounted mixing implements, the Froude number is defined as:

number

[0010] The particulate solids may be metered into the product stream in the form of a dispersion in a gas stream.

[0011] The present invention is based on the finding that uninterrupted metering of particulate solids, in particular aluminium trihydroxide, into the cooler (contact dryer 2) is possible with little effort. Adding the particulate solids directly to the product stream falling into the cooler allows uninterrupted mixing.

[0012] The temperature of the superabsorbent particles during spray application of the surface postcrosslinking agent solution is preferably 30 to 80°C, more preferably 35 to 75°C, and most preferably 40 to 70°C.

[0013] The surface postcrosslinking agent solution preferably contains 0.001 to 2% by weight, more preferably 0.01 to 1% by weight, and most preferably 0.03 to 0.7% by weight of the surface postcrosslinking agent, based on the superabsorbent particles in each case. The surface postcrosslinking agent solution preferably further contains 0.5 to 5% by weight, more preferably 1.0 to 4% by weight, and most preferably 1.5 to 3% by weight of water, based on the superabsorbent particles in each case.

[0014] The superabsorbent particles are heated in the contact dryer 1 to a temperature of preferably 110 to 220° C., more preferably 120 to 210° C., and most preferably 130 to 200° C. The residence time of the superabsorbent particles in the contact dryer 1 is preferably 10 to 60 minutes, more preferably 15 to 50 minutes, and most preferably 20 to 40 minutes.

[0015] The connections to the contact dryer 1 and the contact dryer 2 may be slightly heated and / or thermally insulated.

[0016] The amount of particulate solid used is preferably 0.001% to 2.0% by weight, more preferably 0.01% to 1.0% by weight, and most preferably 0.1% to 0.5% by weight, based on the superabsorbent particles in each case. The average particle size of the particulate solid is preferably 0.1 to 100 μm, more preferably 0.5 to 50 μm, and most preferably 1 to 25 μm. The average particle size is the volume-average particle size and can be determined by light scattering. A preferred particulate solid is aluminum trihydroxide.

[0017] The temperature of the superabsorbent particles when coated with the particulate solid is preferably below 180°C, more preferably below 160°C, and most preferably below 140°C.

[0018] The superabsorbent particles are cooled in the contact dryer 2 to a temperature of preferably 30 to 80° C., more preferably 35 to 70° C., and most preferably 40 to 60° C. The residence time of the superabsorbent particles in the contact dryer 2 is preferably 10 to 60 minutes, more preferably 15 to 50 minutes, and most preferably 20 to 40 minutes.

[0019] The mixing tool of the contact dryer 2 preferably has a diameter of 0.2 to 2 m, more preferably 0.4 to 1.2 m, and most preferably 0.6 to 1.2 m. The speed of the mixing tool is preferably less than 25, more preferably less than 20, and most preferably less than 10 revolutions per minute. DETAILED DESCRIPTION OF THE INVENTION

[0020] The manufacture of superabsorbents is described in detail below.

[0021] Superabsorbents are made by polymerizing a monomer solution and are typically water-insoluble.

[0022] The ethylenically unsaturated monomers having acid groups are preferably water-soluble, i.e., their solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, more preferably at least 25 g / 100 g water, and most preferably at least 35 g / 100 g water.

[0023] Suitable monomers are, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particularly preferred is acrylic acid.

[0024] Ethylenically unsaturated monomers containing acid groups are typically partially neutralized. Neutralization is carried out at the monomer stage. This is typically carried out by mixing a neutralizing agent as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably 40 to 85 mol %, more preferably 50 to 80 mol %, and most preferably 60 to 75 mol %. Customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates, or mixtures thereof. Instead of alkali metal salts, ammonium salts can also be used. Particularly preferred alkali metals are sodium and potassium, but sodium hydroxide, sodium carbonate, or sodium bicarbonate, or mixtures thereof, especially sodium hydroxide, are very particularly preferred.

[0025] The monomer typically contains a polymerization inhibitor, preferably a hydroquinone monoether, as a storage stabilizer.

[0026] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Such groups include, for example, a free radical polymerizable ethylenically unsaturated group in the polymer chain and a functional group that can form a covalent bond with the acid group of the monomer. In addition, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.

[0027] Suitable crosslinking agents include, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, diacrylates and triacrylates, as described in DE 03 / 104301 A1, WO 03 / 104301 A1, and DE 10331450 A1; mixed acrylates which contain further ethylenically unsaturated groups as well as acrylate groups, as described in DE 10331456 A1, DE 10355401 A1, or crosslinker mixtures, as described, for example, in DE 19543368 A1, DE 19646484 A1, WO 90 / 15830 A1, and WO 02 / 032962 A2.

[0028] The amount of crosslinker is preferably 0.05% to 1.5% by weight, more preferably 0.1% to 1% by weight, and most preferably 0.15% to 0.6% by weight, calculated in each case based on the total amount of monomer used. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases, reaching 21.0 g / cm. 2 The absorbency exceeds the maximum value under pressure (AUL 0.3 psi).

[0029] The initiator used can be any compound that generates free radicals under polymerization conditions, such as a thermal initiator, a redox initiator, or a photoinitiator. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Mixtures of thermal and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. The reducing component used is preferably the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, and sodium bisulfite. Such mixtures are available as Brueggolite® FF6 and Brueggolite® FF7 (Brueggemann Chemicals; Heilbronn; Germany).

[0030] The water content of the monomer solution is preferably 40% to 75% by weight, more preferably 45% to 70% by weight, and most preferably 50% to 65% by weight. If the water content is high, the energy consumption in the subsequent drying will increase, and if the water content is low, the heat of polymerization may simply not be sufficiently removed.

[0031] The temperature of the monomer solution is preferably 10 to 90°C, particularly preferably 20 to 70°C, and very particularly preferably 30 to 50°C.

[0032] To function optimally, preferred polymerization inhibitors require dissolved oxygen. Therefore, the monomer solution may be inerted, i.e., flushed with an inert gas, preferably nitrogen or carbon dioxide, to remove dissolved oxygen prior to polymerization. Preferably, the oxygen content of the monomer solution is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, and most preferably less than 0.1 ppm by weight prior to polymerization.

[0033] Suitable reactors for polymerization are, for example, kneader reactors or belt reactors. In kneaders, as described in WO 2001 / 038402 A1, the polymer gel formed during polymerization of an aqueous monomer solution or suspension is continuously comminuted, for example, by a counter-rotating stirrer shaft. Polymerization on a belt is described, for example, in DE 3825366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a belt reactor forms a polymer gel, which must be comminuted, for example, in an extruder or kneader.

[0034] To improve drying performance, the ground polymer gel obtained by the kneader may be further extruded.

[0035] The polymer gel is then dried, typically by an air-circulating belt dryer, until the residual moisture content is preferably 0.5% to 10% by weight, more preferably 1% to 7% by weight, and most preferably 2% to 5% by weight, as measured by EDANA Recommended Test Method No. WSP230.2-05, "Mass Loss on Heating." If the residual moisture content is too high, the glass transition temperature T of the dried polymer gel may be increased. g If the residual moisture content is too low, the dried polymer gel will be too brittle, and the subsequent grinding step will produce an unnecessarily large amount of overly small superabsorbent particles ("fines"). The solids content of the polymer gel before drying is preferably 25% to 90% by weight, more preferably 35% to 70% by weight, and most preferably 40% to 60% by weight. The dried polymer gel is then ground, and optionally coarsely pulverized.

[0036] The dried polymer gel is then typically crushed and classified, and the equipment used for crushing may typically be a single or multi-roll mill, preferably a two or three-roll mill, a pin mill, a hammer mill or a vibratory mill.

[0037] The average particle size of the superabsorbent particles removed as the product fraction is preferably 150 to 850 μm, more preferably 250 to 600 μm, and most particularly 300 to 500 μm. The average particle size of the product fraction can be measured by EDANA recommended test method No. WSP220.2(05) "Particle Size Distribution," in which the mass fractions of the selected fractions are plotted in cumulative form and the average particle size is determined from the graph. In this specification, the average particle size is the mesh size value resulting from a cumulative weight of 50%.

[0038] The properties of superabsorbent particles can be further improved by thermal surface postcrosslinking.Suitable surface postcrosslinking agents are compounds that contain a group that can form a covalent bond with at least two carboxylate groups of superabsorbent particles.Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0083022A2, EP 0543303A1 and EP 0937736A2, difunctional or polyfunctional alcohols, as described in DE 3314019A1, DE 3523617A1 and EP 0450922A2, or β-hydroxyalkylamides, as described in DE 10204938A1 and U.S. Pat. No. 6,239,230.

[0039] In a preferred embodiment of the present invention, in addition to the surface postcrosslinker, a multivalent cation is applied to the particle surface.

[0040] Polyvalent cations that can be used in the process of the present invention include, for example, divalent cations such as those of zinc, magnesium, calcium, and strontium; trivalent cations such as those of aluminum, iron, chromium, rare earths, and manganese; and tetravalent cations such as those of titanium and zirconium. Possible counterions are chloride, bromide, hydroxide, sulfate, hydrogensulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate, and carboxylates, such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0041] The amount of polyvalent cation used is, for example, 0.001% to 1.5% by weight, preferably 0.005% to 1% by weight, and more preferably 0.02% to 0.8% by weight, in each case based on the polymer.

[0042] The surface postcrosslinking is carried out in such a way that a solution of the surface postcrosslinker is sprayed onto the dried superabsorbent particles. After spray application, the superabsorbent particles coated with the surface postcrosslinker are subjected to thermal surface postcrosslinking.

[0043] The spray application of the solution of surface postcrosslinker is preferably carried out using a mixer with moving mixing implements, such as a screw mixer, a disk mixer, and a paddle mixer.Horizontal mixers, such as a paddle mixer, are particularly preferred, and vertical mixers are particularly preferred.Horizontal mixers and vertical mixers are distinguished by the position of the mixing shaft, that is, horizontal mixers have horizontally mounted mixing shafts, and vertical mixers have vertically mounted mixing shafts.Suitable mixers are, for example, horizontal Pflugschar® plowshare mixers (Gebr. Loedige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV; Doetinchem; the Netherlands), Processall Mixmill mixers (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; the Netherlands).However, it is also possible to spray the solution of surface postcrosslinker in a fluidized bed.

[0044] The surface postcrosslinking agent is typically used in the form of an aqueous solution. The penetration depth of the surface postcrosslinking agent into the superabsorbent particles can be adjusted by the content of the non-aqueous solvent and the total amount of the solvent.

[0045] Thermal surface post-crosslinking is carried out in contact dryer, more preferably in paddle dryer, most preferably in disc dryer.Suitable dryer is for example Hosokawa Bepex® horizontal paddle dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® disc dryer (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryer (Metso Minerals Industries Inc.; Danville; USA) and Nara paddle dryer (NARA Machinery Europe; Frechen; Germany).

[0046] The surface postcrosslinked superabsorbent particles can then be classified again to remove overly small and / or overly large superabsorbent particles and reuse them in the process.

[0047] The properties of the surface postcrosslinked superabsorbent particles can be further improved by coating or rewetting.

[0048] Remoistening is preferably carried out at 30 to 80°C, more preferably 35 to 70°C, and most preferably 40 to 60°C. At excessively low temperatures, the superabsorbent particles tend to form agglomerates, while at higher temperatures, water evaporates quickly and noticeably. The amount of water used for remoistening is preferably 1 to 10% by weight, more preferably 2 to 8% by weight, and most preferably 3 to 5% by weight. Remoistening increases the mechanical stability of the superabsorbent particles and reduces their tendency to become electrostatically charged. Remoistening is advantageously carried out in a cooler after hot surface crosslinking.

[0049] Suitable coatings for improving swelling rate and gel bed permeability (GBP) include, for example, inorganic inert materials such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. Suitable coatings for dust adsorption include, for example, polyols. Suitable coatings for combating the undesirable tendency of superabsorbent particles to caking include, for example, fumed silica such as Aerosil® 200, precipitated silica such as Sipernat® D17, and surfactants such as Span® 20. [Example]

[0050] Example 1 (present invention) A monomer solution was prepared by successively mixing deionized water, 50% by weight sodium hydroxide solution, and acrylic acid to a degree of neutralization corresponding to 71.0 mol %. The water content of the monomer solution was 60.5% by weight.

[0051] The crosslinking agent used was 3-ethoxylated glyceryl triacrylate (purity about 85 wt%), and the amount used was 1.42 kg per ton of monomer solution.

[0052] Free radical polymerization was initiated using 0.91 kg of a 0.25 wt % aqueous hydrogen peroxide solution, 4.30 kg of a 15 wt % aqueous sodium peroxodisulfate solution, and 0.84 kg of a 1 wt % aqueous ascorbic acid solution per ton of monomer solution.

[0053] The monomer solution was added to 6.3 ml 3 The monomer solution was introduced into a List Contikneter continuous kneading reactor (LIST AG, Arisdorf, Switzerland) with a capacity of 1000 t / h. The throughput of the monomer solution was approximately 22 t / h. The feed temperature of the reaction solution was 23.5°C.

[0054] The monomer solution was inerted with nitrogen between the addition points of the crosslinker and the hydrogen peroxide and sodium peroxodisulfate solutions. Ascorbic acid was metered directly into the reactor.

[0055] After about 50% of the residence time, about 1000 kg / hour of additional superabsorbent particles having a particle size of less than 150 μm and obtained in a manufacturing process by grinding and classification were metered into the reactor. The residence time of the reaction mixture in the reactor was about 15 minutes.

[0056] The resulting polymer gel was applied to the conveyor belt of an air-circulating belt dryer by vibrating the conveyor belt. The length of the air-circulating belt dryer was 48 m. The effective width of the conveyor belt of the air-circulating belt dryer was 4.4 m. On the air-circulating belt dryer, the aqueous polymer gel was dried by exposing it to a continuous flow of air / gas mixture (approximately 175°C). The residence time in the air-circulating belt dryer was 37 minutes.

[0057] The dried polymer gel was crushed in a three-roll mill and sieved to particle sizes of 150-850 μm. Superabsorbent particles with a particle size of less than 150 μm were separated. Superabsorbent particles with a particle size of more than 850 μm were recycled for crushing. Superabsorbent particles with a particle size in the range of 150-850 μm were thermally surface postcrosslinked.

[0058] Superabsorbent particles were coated with the surface postcrosslinker solution in Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, the Netherlands) and then thermally surface postcrosslinked in a NARA paddle dryer (Contact Dryer 1, GMF Gouda, Waddinxveen, the Netherlands) at 120°C for 45 minutes.

[0059] The following amounts were weighed into a Schugi Flexomix®: 9.5t / hour of superabsorbent particles 530.10 kg / hour of surface postcrosslinker solution

[0060] The surface postcrosslinker solution consisted of 1.43 wt% ethylene glycol diglycidyl ether, 44.8 wt% propane-1,2-diol and 53.77 wt% water.

[0061] The surface-postcrosslinked superabsorbent particles were transferred by a star feeder to a NARA paddle cooler (contact dryer 2, GMF Gouda, Waddinxveen, the Netherlands) and cooled to about 60°C. The surface-postcrosslinked superabsorbent particles were thereby coated with a mixture of about 285 kg / h of water and 23.75 kg / h of an aqueous solution of 1% by weight of sorbitan monolaurate (Span® 20). This mixture was injected into the mixer trough from below the product bed. The distance of the metering point from the end wall was about 200 cm. The diameter of the paddle was about 0.9 m. The paddle rotated at about 10 revolutions per minute. The residence time was about 20 minutes.

[0062] The temperature of the product stream dropping from the star feeder into the NARA paddle cooler was approximately 120°C. The connecting pipe between the star feeder and the NARA paddle cooler was extended to the NARA paddle cooler using an insert tube. The insert tube had a diameter of approximately 20 cm and a total length of approximately 50 cm, of which 30 cm protruded into the NARA paddle cooler. The insert tube should not protrude too far to avoid contact with the mixing equipment inside the NARA paddle cooler. At the same time, the insert tube in the NARA paddle cooler should not be too short to prevent some of the aluminum trihydroxide from being discharged with the output air through the gas space and to retain the maximum amount of aluminum trihydroxide in or on the product surface. A mixture of approximately 33.25 kg / h of aluminum trihydroxide (dried aluminum hydroxide gel, Dr. Paul Lohmann GmbH KG, Emmerthal, Germany) and 45 kg / h of air was metered into the dropping product stream through the insert tube approximately 20 cm from its lower end. The aluminum hydroxide had an average particle size of about 20 μm.

[0063] The coating with aluminum trihydroxide was carried out without interruption.

[0064] Example 2 (not according to the invention) The procedure was the same as in Example 1. The aluminum trihydroxide and air mixture was injected into the mixer trough from below the product bed. The distance of the metering point from the end wall was about 180 cm.

[0065] A blockage occurred when feeding aluminum trihydroxide into the NARA paddle chiller.

Claims

1. 1. A process for continuously producing superabsorbents by coating superabsorbent particles by spray application of a surface postcrosslinker solution, thermally surface postcrosslinking the coated superabsorbent particles in a contact dryer 1, and cooling the thermally surface postcrosslinked superabsorbent particles in a contact dryer 2, wherein the surface postcrosslinked superabsorbent particles are further coated with the particulate solids, which are metered into the product stream between contact dryer 1 and contact dryer 2, contact dryer 2 having two horizontal shafts with mixing devices, the speed of which corresponds to a Froude number of 0.005 to 0.

25.

2. 10. The process of claim 1, wherein the speed of the mixing implement corresponds to a Froude number of 0.04 to 0.

15.

3. 3. The process according to claim 1 or 2, wherein the residence time of the superabsorbent particles in the contact dryer 2 is between 10 and 60 minutes.

4. A process according to any one of claims 1 to 3, wherein the particulate solid has an average particle size of from 1 to 25 µm.

5. A process according to any one of claims 1 to 4, wherein the amount of particulate solid used is from 0.001% to 2.0% by weight, based on the superabsorbent particles.

6. The process according to any one of claims 1 to 5, wherein the mixing tool has a radius of 0.2 to 2 m.

7. The process of any one of claims 1 to 6, wherein the speed of the mixing device is less than 25 revolutions per minute.

8. A process according to any one of claims 1 to 7, wherein the temperature of the superabsorbent particles when coated with the particulate solid is below 180°C.

9. A process according to any one of the preceding claims, wherein the superabsorbent particles are cooled in the contact dryer 2 to a temperature of between 30 and 80°C.

10. A process according to any one of claims 1 to 9, wherein the particulate solid used is aluminium trihydroxide.

11. A process according to any one of claims 1 to 10, wherein the superabsorbent used is a partially neutralized and crosslinked polyacrylic acid.

12. The process according to any one of claims 1 to 11, wherein the surface postcrosslinker is capable of forming a covalent bond with the superabsorbent.

13. The process according to any one of claims 1 to 12, wherein the temperature of the superabsorbent particles during the spray application of the surface postcrosslinker solution is between 30 and 80°C.

14. A process according to any one of the preceding claims, wherein the superabsorbent particles are heated in the contact dryer 1 to a temperature of 110-220°C.

15. The process according to any one of the preceding claims, wherein the residence time of the superabsorbent particles in the contact dryer 1 is between 10 and 60 minutes.

Images